The narrow width excimer laser for lithography generally employs a combination of a prism beam expansion system with an echelle grating for linewidth narrowing. The role of the prism beam expansion system is to expand laser light in the horizontal direction. As the linewidth of a lithography machine is inversely proportional to the expansion factor, a technician strives for an expansion factor as large as possible via combination of prisms. In the current technology, for examples, see U.S. Pat. No. 5,978,409 and Chinese Patent CN101581866B, three to four prisms are employed for realizing an expansion factor of 20-30 times. The sizes of the prisms successively grow with the expansion and transmission of the light beam, with the size of the big prism at the end of the expansion system being generally over 100 mm. In the technical field, material for the prism is generally selected from an ultraviolet fusedsilica or calcium fluoride with high transmittance. As optical materials in large size are difficult to fabricate and limited in production quantity, they are expensive. Further, the current technology employs right angle prisms of different specifications and requires different tools or clamps during polishing, and hence, the processing cycle is long and cost is high.